Recently, radar apparatuses are used as, for example, anti-collision devices for motor vehicles. FM-CW (Frequency-Modulated Continuous Wave) radar apparatuses have been known as such a radar apparatus. The FM-CW radar apparatuses are designed to use a frequency-modulated radio continuous wave signal (FM-CW wave signal) as a transmission signal.
As illustrated by a solid line in the graph (a) of FIG. 7, a typical FM-CW radar apparatus transmits, as a radar wave signal, a continuous-wave transmission signal Ss that is frequency modulated by a triangular signal to have a frequency that linearly varies up and down cyclically over time. The FM-CW radar apparatus also receives a return signal, i.e. an echo signal, as a received signal Sr; this echo signal has been generated based on reflection of the transmitted signal Ss from a target as the received signal Sr.
As illustrated by a dashed line in the graph (a) of FIG. 7, the received signal Sr is delayed with respect to the transmission signal Ss by time Tr; the time Tr is required for the radar wave signal to make a round trip between the radar and the target. That is, the delay time Tr depends on the distance between the radar and the target. In addition, the received signal Sr is shifted in frequency with respect to the transmission signal Ss; this frequency shift, referred to fd, represents the Doppler shift of the received signal Sr with respect to the transmitted signal Ss. The Doppler shift fd depends on the relative speed between the target and the radar.
Mixing the received signal Sr and the transmitted signal Ss together by a mixer generates a beat signal B that is comprised of frequency components of the difference between the received signal Sr and the transmission signal Ss (see the graph (b) of FIG. 7). A frequency of the beat signal B during the period which the frequency of the transmission signal Ss rises will be referred to as a frequency fb1 for up-modulation frequency, and a frequency of the beat signal B during the period which the frequency of the transmission signal Ss falls will be referred to as a frequency fb2 for down-modulation frequency. The period during which the frequency of the transmission signal Ss rises will be expressed as an up-modulation period, and the period during which the frequency of the transmitted signal Ss falls will be expressed as a down-modulation period.
Using the frequencies fb1 and fb2 for up-modulation and down-modulation allows a frequency fr based on the delay time Tr to be expressed by the following equation [1], and the Doppler shift fd to be expressed by the following equation [2]:
                    fr        =                                            fb              ⁢                                                          ⁢              1                        +                          fb              ⁢                                                          ⁢              2                                2                                    [        1        ]                                fd        =                                            fb              ⁢                                                          ⁢              1                        -                          fb              ⁢                                                          ⁢              2                                2                                    [        2        ]            
Using the frequencies fr and fd allows the distance R between the target and the radar and the relative speed V therebetween to be expressed by the following equations [3] and [4]:
                    R        =                              c            ·            fr                                              4              ·              fm              ·              Δ                        ⁢                                                  ⁢            F                                              [        3        ]                                V        =                              c            ·            fd                                2            ·            Fo                                              [        4        ]            
where c represents the propagation speed of radio waves, fm represents a modulation frequency fm of the triangular signal, ΔF represents a variation in frequency of the transmission signal Ss, and Fo represents a center frequency of the transmitted signal Ss.
In order to identify the frequencies fb1 and fb2 for up-modulation and down-modulation, values of the beat signal B are sampled at a sampling frequency. Sampled values of the beat signal B during the up-modulation period are subject to a Fast Fourier Transform (FFT), so that frequency components of the beat signal B for up-modulation are obtained. Similarly, sampled values of the beat signal B during the down-modulation period are subject to a FFT, so that frequency components in strength of the beat signal B for down-modulation are obtained.
A peak frequency component in strength of the beat signal B during the up-modulation period is extracted, as the frequency fb1, from the frequency components in strength of the beat signal B for up-modulation. Similarly, a peak frequency component in strength of the beat signal B during the down-modulation period is extracted, as the frequency fb2, from the frequency components in strength of the beat signal B for down-modulation.
As is well known, the sampling frequency fs should be set to be twice or more the upper limit frequency of the beat signal B. That is, the variation in frequency of a frequency-modulated transmission signal Ss to be transmitted from the radar and a modulation period 1/fm of the triangular signal should be determined such that frequency components of beat signals B, which will be generated based on echo signals from any targets existing in a predetermined detection range of the radar, are within the effective frequency range defined for detection of the beat signals B; this effective frequency range is equal to or lower than the upper limit frequency.
There have been various approaches to eliminate noise converted by the mixer into frequency components within the effective frequency range defined for detection of the beat signals B, i.e. noises with their frequencies close to the frequency of a radar wave signal to be transmitted as a transmission signal. One example of these known noise-elimination approaches is disclosed in US Patent Application Publication No. 2008/0231496A1 corresponding to Japanese Patent Publication No. 4356758.